Asian insect imports meet in Virginia, North Carolina soybeans

• Since the north to south movement of brown marmorated stink bugs began, researchers have tracked their relatively slow migration.
• In 2009, when kudzu bugs were first found in north Georgia and began a rapid south to north movement, the tracking game took on a new intensity.

The extent of damage caused by brown marmorated stink bugs and kudzu bugs in Virginia is not known for certain, but having both Asian imports meet in several counties in Virginia is cause enough for entomologists and growers in both North Carolina and Virginia to take notice.

Since the north to south movement of brown marmorated stink bugs began, researchers have tracked their relatively slow migration. In 2009, when kudzu bugs were first found in north Georgia and began a rapid south to north movement, the tracking game took on a new intensity.

Kudzu bugs rapidly infested both Carolinas and a few made it to Virginia in 2011.

This year larger, though as of yet not damaging numbers of kudzu bugs, have shown up in Virginia. Figuring out where the overlap of the two insects would meet has been an ongoing challenge, but it appears the first duel battleground will be in southeast Virginia and northeast North Carolina.

Brown marmorated stink bugs (BMSB) have been around for more than 10 years, first showing up in Pennsylvania in about 2000. In the past few years, they appear to have made a determined movement southward into Maryland, Delaware and Virginia, and most recently into North Carolina.

Though the BMSB spread southward has been slow as insect migrations usually go, this particular stink bug has created some widespread and sporadic problems for farmers.

Perhaps the biggest problem has been in the grape and fruit production areas of northern Virginia. Wine grape growers in particular have had a difficult time with the foul-smelling insects when they burrow down into the whorl of grape plants.

In the past three years, BMSB have been found in soybeans in Virginia. In 2011, the infestations were heavy at times, but generally confined to the outer rows of soybean fields. This year these insects were found throughout much of Virginia, a big spread in a year’s time for this particular insect, but infestations were not heavy in any one area.

Virginia Tech Entomologist and State IPM Leader Ames Herbert has been tracking the movement of these insects and studying their biological traits for the past three years.

Later movement

With the warm winter across the Upper Southeast there were some concerns that BMSB may occur earlier than in previous years, but that didn’t happen. If anything movement of these insects was later this year and we don’t understand BSMB well enough to know why they were slower to start occurring in soybean fields this year.

The Virginia Tech Entomologist says once the new stink bugs began to show up, they spread across the state much faster than in previous years. “Once they started happening, they started popping up everywhere,” Herbert says.

Virginia only grows soybeans in 70-75 counties and BMSB have now been documented in more than 40 of these counties. “We are already seeing ‘stay green syndrome’ damage on the edge of soybean fields, indicating damage from BMSB was there and the grower didn’t pick it up,” Herbert notes.

The majority of damage we see this year is along the edges of soybean fields, but again they threw us a curveball by moving farther into some fields. A big change this year was that we never found BMSB alone — they had always occurred in mixed populations with native stink bugs.

In cage studies conducted in cotton fields, researchers have found BMSB not only like cotton, but that they are physically able to cause more damage than their native cousins, brown and green stink bugs.

“We are concerned because we have now documented BMSB in at least two counties in Virginia in which both cotton and soybeans are grown. One BMSB was in cotton, though both were in cotton producing counties, and we are concerned that BMSB will become a problem in cotton in coming years,” Herbert says.

Cotton and brown marmorated stink bugs would be a bad combination for growers. Herbert says BMSB appear to prefer older, larger bolls. In some cases they did damage last year to bolls that in the past we have considered safe from native species. If this year’s data shows the same trend toward BMSB damage to older, bigger bolls, then we will have to be really more concerned about cotton, Herbert says.

The BMSB species has a longer and stronger feeding beak. In corn, for example, these insects can easily penetrate the husk and do great damage to corn kernels.

“That’s probably what’s happening with cotton. They just simply are better equipped than native species to penetrate older cotton. In soybeans, the damage is not different, but penetrating a soybean pod is easy for native species,” Herbert says.

In North Carolina, North Carolina State University Entomologist Jack Bacheler found a few BMSB in some soybean fields in areas bordering Virginia.

Treating for kudzu bugs

Soybean growers are already treating for kudzu bugs in some cases and having BMSB would just be one more problem for them to deal with next year.

A reoccurring problem with both BMSB and kudzu bugs is a lack of long-term data that can be applied to management strategies.

“Native stink bugs have caused problems in crops for a long, long time, yet we still don’t have a good handle on how to manage them. The new species is different, and it’s really difficult to make good management recommendations based on such a short-term of study, Herbert says.

Kudzu bugs have been around an even shorter time than BMSB, making the mixing of both species in production agriculture double-trouble. Like the BMSB, kudzu bugs are distantly related to native brown, green and Southern green stink bugs that are commonly found in a multitude of crops in the Southeast.

And, like their cousins, kudzu bugs and BMSB are relatively easy to kill — that’s not the problem. Knowing at what population to spray, how to spray and when to spray can be big problems with either of the two new species.

Since appearing in northeast Georgia in 2009, kudzu bugs now blanket the Southeast from Florida, north to Virginia and west to Alabama, with pockets of infestations reported in counties in Tennessee and Mississippi. The insect seems to reproduce and spread as efficiently as kudzu.

"It will probably spread and survive anywhere kudzu survives," says Joe Eger of Dow Agrosciences, speaking at a recent South Carolina field day. Eger showed attendees a map showing kudzu's range in the U.S. — from Texas and Nebraska to New York and Massachussetts.

"So far kudzu bugs are found almost anywhere kudzu is found, so I suspect it will go much farther north," Eger says.

Even at the end of the growing season this year, entomologists across the Southeast were still getting calls about kudzu bugs at the R6 growth stage. By this time of the year many growers are finishing up with corn, defoliating cotton and digging peanuts and scouting soybeans for kudzu bugs in the last thing they want to do.

Direct bearing on bottom line

Then again, most of those beans were booked at more than $15 a bushel and keeping yields high has a direct bearing on the bottom line. Management decisions like these are going to get more and more difficult as both these new insects from Asia continue to march on toward one another.

“The unfortunate fact is that many of these fields where kudzu bug is above threshold should have been sprayed a month ago. With R7 as our “safe stage” from kudzu bug and other defoliating pests, it becomes a hard call to know whether to spray kudzu bugs in beans at R6,” says Clemson Entomologist Jeremy Greene.

Things to consider are how long the insects may have been out there, which is hard to know without scouting the fields. Knowing how close the field is to R7 and how much yield can be lost by running a sprayer through a field of soybeans at the R7 growth stage is likewise hard to know.

The challenge is magnified when a grower is evaluating a soybean field in which $15-plus per bushel beans are growing, when R7 is determined by one brown pod on a majority of plants in a field.

Speaking at a recent regional Kudzu Bug Field Day held at the Edisto Agricultural Research and Education Center in Blackville, S.C., Greene says about kudzu bugs, “The good news is, we can kill them. The bad news is that killing them will cost us, and the damage they do in the meantime will cost us even more.”

Bacheler says in North Carolina, “In general our results align with those from Georgia and South Carolina. Pyrethroids and pre-mix products containing pyrethroids are very effective. Remember to stay away from anything with cyfluthrin. Some of these products look good at reduced rates.”

As more and more BMSB move southward and mix with already high populations of kudzu bugs, problems for growers are sure to get worse. The same insecticides will likely work on both species, but sampling methods, thresholds, etc, may be very different.

The trick will be to find the right combination of thresholds, insecticides and other management strategies, and these will come with time.